Frequency-modulated crystal oscillator



July 22, 1947. w. P. MASON 2,424,246

FREQUENCY-MODULATED CRYSTAL OSCILLATOR Filed Sept. 16, 1943 2 SheetsSheet 1 vFIG.

FIG. 4

REA 6 TANCE mill 1 I INVENTOR. By W P MASON ATTORNEY a 303/ 4 J2 H68 1 Patented July 22, 1947 FREQUENCY-MODULATED CRYSTAL OSCILLATOR Warren P. Mason. West Orange N. J.,v assignmto Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 16, 1943, Serial No. 502,560

4: Claims.

This invention relates to frequency-modulating systems and particularly to frequency-modulating systems and apparatus that may be utilized for varying or modulating the frequency of quartz or other crystal-controlled oscillation generators, for example, or of other types of crystalcontrolled frequency-modulated circuits.

One of the objects of this invention is to control the operating mechanical characteristics of a vibratory piezoelectric crystal element, such as av quartz or other type of piezoelectric crystal element.

Another object of this invention is to utilize a piezoelectric crystal element for varying or modulating the frequency of the circuit in which it may be connected.

Another object of this invention is to provide a simple method for frequency modulating a crystal oscillator without losing its inherent frequency controlling stability.

In accordance with this invention variable positive reactance means, such as a variable reactance tube and an inductance coil of suitable reactance values, may be connected with and utilized in combination with a vibratory piezoelectric crystal element such as a quartz or other type of piezo electric crystal element. The reactance of the reactance means and the reactance of the crystal element may be made of suitable related values with respect to one another in order to effectively control the operating characteristics and the frequency of the crystal element which may in turn be utilizedto control the frequency of an oscillation generator or any other circuit the frequency of which is to be controlled by the crystal element.

The crystal element and the variable positive reactance tube means may be utilized as a frequency-modulation system for any type of crystal -controlled oscillation generator, and the same crystal element may be utilized not only for controlling the frequency of the oscilation generstar but also for modulating the frequency thereof in accordance with rectified voice or other signal currents, the reactance tube means. being utilized to produce a positive reactance that is controlled and varied. by the amplitude of the signal currents applied thereto in Order to correspondingly change the reactance characteristics of the crystal element,.thereby causing the fre.-. quency of the crystal element and the oscillator generator associated therewith to change in ac cordance with the amplitude of the signal cure rents that may be applied to the input or grid circuit of the reactance tube means.

The frequency-modulation system providedin accordance with this invention may be utilized to. vary or modulate the frequency of a c y talcontrolled oscillation generator over relatively wide frequency limits, without losing the inherent frequency stability of the crystal element. In a particular frequency-modulated crystal oscillator using a split platin form of electroded quartz crystal element with applied voltage bias on the grid of the reactance tube, a crystal frequency variation was obtained of about 15.3 kilocycles per second in a crystal element of about 2,100 kilocycles per second, corresponding to a crystal frequency variation of about 0.73 per cent. Using another setting in the same crystal oscillator gave about a 3 kilocycle per second crystal frequency variation with a more stable output.

For a clearer understanding of the nature of this invention and the additional advantages, features and objects thereof, reference is made to the following description taken in connection with the accompanying drawings, in which like reference characters represent like or similar parts and in which:

Fig. 1 is a circuit diagram illustrating a frequency-modulated crystal oscillator system em-- bodying this invention;

Figs. 2, 3 and 4 are diagrams illustrating characteristics of crystal elements and reactance means utilized in this invention;

Figs. 5, 6 and 7 are graphs illustrating some experimental results obtained in operating a par- I ticular frequency-modulatedcrystal oscillator of the type shown in Fig. 1; and

Fig. 8 is a circuit diagram illustrating a modification of the system shown in Fig. 1.

Referring to the drawing, Fig. 1 is a circuit diagram illustrating an embodiment of the invention as applied to a crystal-controlled oscillation generator l the frequency of which is controlled by a piezoelectric crystal element 2 which may be of any suitable type. As illustrated in Fig. 1, the crystal element 2 may be provided with two effective pairs or sets of oppositely disposed electrodes or coatings 3, 4, 5 and 6, one set of electrodes consisting of the opposite electrodes 3 and 4 be, ing utilized to control the frequency of the oscillation generator I, as shown in Fig. 1. The par, ticular illustrative example given in Fig. 1 for the oscillator generator I comprises a triode oscillator tube 8, a coupling condenser 9, suitable voltage supply sources In and l l for the cathode and plate electrodes of the vacuum tube 8, a gridcathode resistance l2, a tuned output circuit which may consist of a parallel-connected con,-

oscillator tube 8. The by-pass condenser I is battery sour i the oscillation gener shunted across the plate supply It will be understood that ator circuit l of Fig. 1 may be any suitable oscillator circuit the frequency of whichis to be controlled by the piezolectric crystal element -2 connected therewith in any suitable manner, the particular oscillation generator circuit l shown in Fig. 1 being an illustrative example only.

The piezoelectric crystal element 2 may be any suitable quartz crystal element or may be any other type of piezoelectric crystal element such as a crystal element of tourmaline, Rochelle salt, or ammonium dihydrogenphosphate, for example. The frequency of the crystal element 2 may be made of any desired value, and may be determined either by the thickness dimension thereof or by-the large or face dimensions thereof according to the particular crystal element selected. As an example for a relatively low frequency facemode quartz crystal element, the crystal element 2 may be 2. GT cut crystal element of the type disclosed, for example, in W.P. Mason Patent 2,204,- 762, dated June18, 1940. As an example for a relatively high frequencythickness-mode quartz crystalelement, the crystal element 2' may be an AT or-BT cut crystal element'of the typedisclosed' for example in Lack et a1; "Patent 2,218,200, dated October 15, 1940, and R. A. Sykes Patent'2,306,909, dated December 29., 1942. The GT cut quartz crystal element asdisclosed in W. P. Mason Patent 2,204,762, dated June 18, 194:0, may be utilized when a'relatively low, frequency crystal element is desired, and has the advantage of very high temperature-frequency stability.- The AT or BT cut crystal elements are low' temperature-frequency coefiicient crystal elements which may be utilized where a higher frequency crystallelement is desired. .It will be understood, however,

that'the crystal element 2 of Fig. 1 may be any suitable piezoelectric crystal element.

As illustrated in Fig. 1, the crystal element 2 is provided with two sets of oppositely dis-.

posed electrodes 3, 4, 5 and 6, which may be formed integral with the quarts or other crystal body'2 and which may be composed of any suit able conductive material such as evaporated gold or silver, for example. The two lower electrodes 4 and 6, instead of being divided or split along the center line of the major face of the crystal element 2 as illustrated in Fig. 1 may be united into a single electrode coating and used with the two separated top electrodes 3 and 5 to form the two effective sets of electrodes for the crystal element 2.

As illustrated in Fig. 1, one set of the crystal electrodes comprising the set of opposite electrodes 3 and 4 is connected with the grid. and cathode electrodes of the oscillator tube 8 and functions to control the frequency of oscillations generated in the oscillation generator I.' A variable reactance tube20 and an inductance coil 2| are connected with the second set of crystal electrodes 5 and 6 and function to control and modulate the resonant frequency of the crystal element 2,.the resonant frequency of which may be conven- I3 and inductor [4 being connect til 4 iently varied by some 0.5 per cent or more as described more fully hereinafter. In order to maintain the non-modulated frequency of the crystal element 2 constant to around some five parts in a million, the grid bias of the reactance tube may be held constant to about one part in a thousand'by means of a stable source of substantially constant voltage '22.

The reactance tube 20 may be provided with a suitable plate supply voltage from a source 23,

and with cathode current from a supply source 24. A grid resistance is'connected with the grid of the reactance tube 20. Condensers 25 and 2 'I are by-pass condensers connected in shunt cir- -'-cuit relation with the voltage supply sources 22 and-23 respectively. Condensers 28 and 29 and a resistance are also provided. The resistance 30 may be connected to a tap 3! on the coil 2!. The

condenser 28 functions to prevent the direct current voltage of the source 23 from being applied to the grid of the tube 20. :The resistance 30 and condenser 29 function to produce the correct phase on the grid of the tube 20. The connections of the component elements involved in the input and the output circuits of the reactance tube 20 are as follows: The set of crystal electrodes 5 and 0 are connected across the plate-cathode or output circuit of thereactance tube 20. The inductance coil 2| connected in series circuit relation with the by-pass condenser 21 is connected across the output circuit of the reactance tube 20, and, as particularly illustrated in Fig. 1, is disposed in shunt relation with the set of electrodes 5 and 6 of the crystal body'2. The resistor 30 is connected at one of its ends to the tap connection 3| on the inductance coil 2| and at its other end the resistor 30 is connected to the condensers 28 and 29, the condenser 28 being connected between the resistance 30 and the control grid or input electrode of the reactance tube 20 and the condenser 29 being connected at one of its ends to the cathode of the reactance tube 20 and at its other end to thecondenser 28 and the resistance 30. The grid-cathode or input circuit of the reactance tube 20 comprises the grid resistor 25 connected in series circuit relation with the paralleleonnected voltage source 22 and by-pass condenser 26. Currents from a. signal source 33 may be impressed across the input circuit of the reactance tube 20 in any conventional manner, as by means of a series-connected transformer therein for example.

As an illustrative example, the circuit elements ofa particular frequency-modulated crystal oscillator constructed in accordance with the circuit of Fig. 1 may have values roughly as follows: The tubes 8 and 20 may be type 205-D vacuum tubes or other suitable thermionic tubes. The plate supply. sources II and 23 may be of voltsdirect current or of other suitable plate supply voltage for the tubes 8 and 20. The crystal element 2 may be an AT-cut thickness shear modev crystal element having a thickness-mode frequency of about 2,100 kilocycles per second, for example. The condenser l3 and inductance windings l4 may be oi. any suitable values to provide a tuned. output circuit for the oscillation generator 1|. The. grid. resistance l2 for the oscillatortube B'may be about 100,000 ohms or other suitable value. Thegridresistance 25 for the reactance tube.20 may have aresistance value of about one megohm, 'andthe source 22 may be of about.45 volts direct current. The resistance 30 may have a resistance value of about 100,000 ohms, and the condenser 28 may have a capacitance of about .005 microfarad or other suitable value to keep direct current voltage from the source 23 off the grid of the reactance tube 20. The condenser 29 may have a capacitance of about 5 micromicrofarads or other suitable value. The values of the reactances of the inductance coil 2| and of the reactance tube 20 are adjusted to suit the reactance of the static capacitance of the crystal element 2, as more fully described elsewhere in this specification.

The reactance tube 20 may be any suitable vacuum tube which taken with the inductance coil 2| has a positive reactance value which is near to or in the neighborhood of the reactance value of the static capacitance of the crystal element 2 and hence may operate as a modulating reactance to vary the resonant frequency of the crystal element 2 by changing the mechanical properties of the crystal element 2. By the term positive reactance, is meant that the reactance value of the tube 20 and coil 2| as meassured from terminal 3| to ground exhibits a positive reactance similar to that measured for an inductance coil.

The change in the mechanical properties of the split-plated crystal element 2 is due to its termination in positive reactances 20 and 2| of a definite control value. The reactance tube 20 may be used for producing the control value of the reactance and for varying the reactance of the inductance coil 2| in such a way that the mechanical properties of the crystal element 2 are changed, thereby causing the frequency of the crystal element 2 and the frequency of the oscillation generator I to change correspondingly. The modulated reactances 20 and 2| are placed across one set of the opposite crystal electrode coatings 5 and 6 and bein a variable reactance of suitable value with respect to the static reactance of the crystal element 2, the resonance frequency of the crystal element 2 is changed correspondingly.

Fig. '2 is a diagram illustrating the equivalent circuit of the piezoelectric crystal element 2 of Fig. 1 that has two sets of opposite crystal electrodes 3, 4, 5 and 6, one set of which, such as the electrodes 5 and 6, is connected across or in parallel circuit relation with an inductance winding which may be the inductance coil 2| of Fig. 1 and which is designated Loin Fig. 2. The equivalent circuit shown in Fig. 2 for the divided plating crystal element 2 of Fig. 1 consists of a capacitance labeled Co/Z in Fig. 2, which is equal to one-half the static capacitance C of the crystal element 2, an electromechanical transformer of ratio 1 to 44 as shown in Fig. 2, a series resonant circuit designated LM, CM in Fig. 2 which represents the mechanical elements of the piezoelectric crystal element 2 of Fig. 1, another electromechanical transformer 4 5 to 1, and a shunt capacitance Co/2 which represents the set of electrode plates 5 and 6 of the crystal element 2 of Fig. 1. For information on the nature of the transposition ratio 4 reference is made to page 205 of the book Electromechanical Transducers and Wave Filters, D. Van Nostrand, by the applicant (Figs. 6, 7).

As shown in Fig. 2, the electromechanical transformer labeled, 4 to 1 and the shunt capacitance 00/2 representing the crystal circuit for the set of electrodes 5, '6 of the crystal element 2 is terminated in the inductance Lu. Bringing the inductance Lo and the capacitance Co/2 of Fig. 2 through the electromechanical transformer le to 1 of Fig. 2, the resulting network s curve for the combination curves of the two elements comprising the crysta1 element and the inductance Lo are shown in Fig. 4 by the broken lines, while the reactance crystal element and inductance L0 is shown by the solid line curve of Fig. 4.

As shown by Fig. i, when the antiresonance frequency of the capacitance 00/2 and the inductance Lo of Figs.

2 and 3 is above the natural resonance frequency in of the crystal element 2,

the result will be to lower to fa the resonant low the value of fa.

To take advantage of this characteristic, a reactance tube 26 of Fig. 1 may be placed across the crystal element 2 constants thereof a is impressed across the crystal actance variation element 2 of Fig.

coil 2| may be adde reactance tube 26 in electrodes 5 and 6 of the of Fig. 1 and the reactance djusted so that a positive re- 1. In practice, an inductance d and shunted across the order to make the reactance variation applied to the crystal element 2 come in the proper reactance range. Th fied voice or signal currents from en when rectia microphone or other signal source 33 are impressed across the grid input cir cf Fig. 1, the resonant frequency element 2 may be cent or more in p the voice or other applied to the input circuit of the 26. If the crystal or zero temperature coefiicient of ctance tube 20 of the crystal made to change some 0.5 per roportion to the amplitude of signal currents which may be reactance tube e having a low frequency, the

cult of the rea element 2 be on mechanical elements thereof, designated LM and CM in Figs. 2 and 3, will change very little with temperature so that the frequency of the crystal controlled oscillator I of Fig. 1 will be nearly independent of temp a high constancy f erature. In order to maintain or the mean crystal frequency,

in the absence of modulation, it is required that the voltage bias from the direct current source 22 of Fig. l on the g rid electrode of the reactance tube 2i] be maintained at nearly constant value,

in order to provid the crystal element 2.

variation of the crystal age bias changes tube 20 is to be ab e a constant reactance across As an example, if the total frequency caused by volton the grid of the reactance out 0.5 per cent, then in order to maintain a frequency constancy of about 5 parts in a million when no modulati from the due to voltage bias changes on is present, the voltage bias source 22 may be maintained constant to about one part in a thousand.

Fig. 5 is a graph illustrating the change in frequency of a crystal oscillator of the type illustrated at in Fig 1, when operated with variable inductance such as may be provided by an inductance coil 2| electrode platings placed across the secondary 5 and 6 of the crystal element 2. More particularly, Fig. 5 illustrates the efiect of adding Varying one pair of the crystal electrodes 5 the frequency of 0 values of inductance across and 6 upon scillations generated in a particular crystal-controlled oscillation generator I of the type illustrated in Fig. 1.

The particular crystal element 2 used in obtaining the data for the example illustrated by the graph in Fig. 5

was a thickness shear mode AT 0 tal element 2 having two sets ut type of crysof oppositely disposed electrodes formed integrally with the major faces thereof, one

set of electrodes 3 and 4 being nect'ed across an inductance coil such'as the in ductance coil-2t of-Fig. 1. Using the two'sets'ofcrystal electrodes 3; t, 5' and 6 referred to; the tl'iiokness-mode frequency obtained was about 2078 kilocycles per second, which correspondsto the second thickness shear mode ofmotion excitedin such a crystal element 2 by the oscillator I when the electrodes 5- and 6 are opencircuited. When the electrode plates 5 and 6 thereof were short-circuited, the crystal frequency dropped slightly to a value of' about 2077.115 kilocycles per second. The eflect of adding inductance by means of a coil 2-! placed across the crystal electrodes 5 and 6 of the crystal element referred to is shown in Fig. 5. Inductances having reactance values that are smaller than the reactance of the static capacitance of thecrystal element 2 cause the frequency thereof to decrease with increasing values of inductance, asillustrated by the curve A of Fig. 5. Also, inductances having reactance values that are slightly larger than the reactance of the static capacitance of the crystal element 2 cause the upper resonance frequency thereof to decrease withincreasing values of inductance, as illustrated by the curve B of Fig. 5. To obtain a relatively large change of resonant frequency in the crystal element2, it is=necessary to use and vary the reactance of the reactance tube 20 and coil 21 inthe neighborhood of the static reactance value of the crystal element 2, since a reactance change therein at any other value will not have very much effect on the resonant frequency of the crystal element 2. Accordingly, to obtain a relatively large change in the resonant frequency in the crystal element 2 of Fig. 1 by means of a change inbias or modulating voltage applied to the input grid circuit of the reactance tube 20 of Fig- 1, the reactance of the tube 20 may be made to have a value such that the plate impedance of the reactance tube 20 is reasonably low so that the reactance component 01" the reactance tube 20 will then not be too high-to affect the'shunting reactance coil 2| appreciably. For this reason, the reactance tube- 20 may be a low impedance triode 20'or other suitable relatively low impedance tube cooperatingwith-the shunting reactance coil 2|, the reacta rice of which is varied in the neighborhood of the static reactance of the crystal element 2. The static reactance of the crystal element 2, which may have a reactance value of around 4,000 ohms, for example, or other value. will vary according to the particular crystal element selected.

In a particular case, using a particular AT cut crystal element in the circuit arrangement of Fig. 1 provided with a suitable shunting coil 24 and shunting capacitances 28- and 29 for the reactance tube 29; the operating output frequency of the crystal-controlled oscillator followed curve B of Fig. 5. By varying the voltage from the source 22 or other rectified source ofcur 1155 as applied to the grid of the reactance changed as shown in Fig. 6.

Fig. 6 is a graph illustrating the frequency changes and. the grid current changesin apar ticular. frequency-modulated crystal oscillator of the type shown inFig. 1, for varying values of voltage bias applied to the gridcircuit" of the reactance tube 28. With a fixed voltage bias of about 27.5 volts negative applied to the grid of the crystal oscillator frequency maybethe tube 25- from the direct current source of' voltage-2 2 and a variable voltage of about 27.5 voltsabove andbelow the voltage value of 27251 voltsmentioned, a crystal frequency change ofab'o'ut '9 kiloc'y'cl'es per' second was obtained about a are anscrystai frequency 'of about 2130 kilocycles per second} as shown by the curve C of Fig. 6. Ove'ntliis-Sl kilo'cy'cl'es per second frequency range; tlie-g-r id current cftlie oscillatortube 8 does not change more than about six'per cent, as shown by thecurve D- of Fig. 6 sothat theoutput of the oscillator I does not have much amplitude modulationover the modulation frequency range Iflhfiibfie'd.

In -thismethod of frequency modulating a crystal "oscillator, it-will be notedthat a relatively wiuezrrequency variation may be obtained. in the ciyst'aiieiement- 2 by means ofa variation in aepuedvoltage bias on the input" circuit of the reactance tube 202 As shown in'Fig. 6', a crystal element't when used in combination with the're act'a-ncetube 20 and the shunting reactance c'oll 2'1; may have an output circuit frequency varla tion of some 15.3 kilocycles per se'condbetween the crystal frequency values of about 2120" and 2 136 kilocycles pensecond', which represents a crystal frequencyvariation's of some 0.75per cent as obtained w ith voltage bias variationapplied to the grid circuit of the reactance tube 20. Over about 9 kiloo'ycl'es ofthis-15:3 kilocycle per secend range sho'wii i'ri'F igi 6, the crystal frequency variation maybe quite linear with voltage-change, the outputbeing changed but little. This 15.3 kilocycle per second frequency range may be somewhat larger may be required or desired for many frequency modulation" systems andmo'reover' comes at a reactance setting of the reactance tubei'lland reactance coil 21" wherethe frequency or the crystal element 2 depends to quite an" extent upon the reactance value-of the shunting reactancecoil' 21: Accord ingly, another value for the-reactance setting of thereaetance coil 21' may b'edesirable in order to' giveas smaller output frequency change and one that may be more linear with bias voltage change such* as, for" example; a 3" kilo'cycle pe'r second output frequency change iri the crystal frequency of about 2'100' kilocy'cles per second, as illustrated by the particular example given in Fig. 7.

Flg. 7 is a graph illustratingan exampleof a smaller ra ge" in" thecrystal frequency change oi tlie freguency-modulated crystal oscillator circuit of Fig-l when provided with adifferent'set ting the" reactance-values of the'variable'reac't'ance 'ni'eans used with" the same crystal ele inehu as compared with tlio'se values that we're used in-obta'i'riing' the curves-of Fig; 6. The differe'nt settingfreferred to involves the addition of al-slig-ht additional amount of" capacity that may -be provided by a* condenser put in parallel with-the inductance coil 21 of Fig; 1' and which niove's tlie' operation further up the reactance curve, jiesdltiiri'g in a smaller variation of output frequency: with applied variable bias voltage, as showr'rin'Fig, '7' where a.- frequency variation of about 3 kilocycles per second is obtained'with a voltagebiasbfabout 27.5'volts on theg'rid'of the tube Zlfletndiabout:t'lz5 volts' change therein. As shown i Fig. '7; the crystal output frequency change is* linear overa" range of about 2.5'kilocycles of the three kilocycles per second" range referred to; and the average frequency is quite 'stablewith small changes in the reactance of tube 20, and similar small changes in other circuit parameters. To hold the mean frequency within about 100 parts in a million, for example, requires a coil and condenser stability of about 0.25 per cent. Since coil and condenser temperature coefilcients are of the order of about 30X per degree centigrade, this would allow about a lilo-degree centigrade change. When tubes are changed, it may be desirable to adjust any resulting capacity difiererrce by means of a small variable condenser. The voltage bias applied to the grid of the tube 20 from the direct current battery source 22 should be kept constant to within about 1% of a volt or less for stability in the mean operating frequency of the crystal element 2.

Fig. 8 is a circuit diagram showing a modification of the frequency-modulated crystal controlled oscillation generator illustrated in Fig. modified to provide the crystal element 2 with a single pair of electrodes 3a and 4a which are connected in the grid circuit of the oscillator tube 8 in series with the inductance coil 2| provided with the proper value of reactance. The reactance of the inductance coil 2| of Fig. 8 is varied by means of the reactance tube '20 in the manner heretofore described in connection with Fig. 1. As in the case of the arrangement illustrated in Fig. 1, the reactance of the inductance coil 2| of Fig. 8 is made to have very nearly the same reactance as the static capacitance of the crystal element 2 in order to obtain the optimum effect of frequency variation in the crystal element 2 with variations in the voltage bias applied to the grid of the reactance tube 20 from the source 22. A condenser 32 may be connected in series circuit relation with the crystal element 2 and the inductance coil 2| and may be utilized for the purpose of obtaining a fine adjustment of the mean frequency of the oscillator.

Although this invention has been described and illustrated in relation to specific arrangements, it is to be understood that it is capable of application in other organizations and is therefore not to be limited to the particular embodiments disclosed.

What is claimed is:

1. Apparatus for varying and controlling the frequency of oscillation of an oscillation generator comprising a piezoelectric crystal body having not more than two efiective sets of electrodes, said electrodes comprising the sole effective capacitance producing means directly shunted across said crystal body, and means including said crystal body controlled by positive reactance means having a definite predetermined control value responsive to the amplitude of variable bias potential applied thereto for correspondingly varying the operating' frequency of. said crystal body and said oscillation generator, said positive reactance means comprising a variable reactance tube having anode, cathode and control grid electrodes, and an inductance coil disposed in circuit relation across said anode and cathode electrodes of said reactance tube and connected in operative circuit relation with said electrodes of said crystal body, said inductance coi having a tap connection thereon, a resistor connected at one of its ends to said tap connecticn on said inductance coil, and a condenser connected between the other end of said resistor and said cathode electrode of said reactance tube, and another condenser connected between said other end of said resistor and said control grid electrode of said reactance tube, said inductance coil having an inductive reactance substantially greater than the anode circuit impedance of said reactance tube and substantially equal to the static capacitive reactance of said crystal body at said operating frequency of said crystal body, and means including a stable source of constant bias potential disposed in circuit with said control grid and cathode electrodes of said reactance tube for providing a constant mean value for said non-modulated frequency of said crystal body.

2. Apparatus in accordance with claim 1 wherein said crystal body has two effective sets of electrodes, one set of said two sets of electrodes being disposed in shunt circuit relation with said inductance coil and the other set of raid two sets of electrodes being disposed in the grid-cathode input circuit of said oscillation generator.

3. Apparatus for varying and controlling the frequency of oscillation of an oscillation generator comprising a piezoelectric crystal body having a single pair of opposite electrodes, said electrodes comprising the sole effective capacitance producing mean directly shunted across said crystal body, and means including said crystal body controlled by positive reactance means responsive to the amplitude of variable potential applied thereto for correspondingly varying the operating frequency of said crystal body and said oscillation generator, said positive reactance means comprising a variable reactance tube having anode, cathode and control grid electrodes, and an inductance coil disposed in circuit relation across said anode and cathode electrodes of said reactance tube and connected in series circuit relation with said single pair of electrodes of said crystal body, said inductance coil having a tap connection thereon, a resistor connected at one of its ends to said tap connection on said inductance coil, and a condenser connected between the other end of said resistor and said cathode electrode of said reactance tube, and another condenser connected between said other end of said resistor and said control grid electrode of said reactance tube, said inductance coil having an inductive reactance substantially equal to the static capacitive reactance of said crystal body at said operating frequency of said crystal body, and means including a stable source of constant bias potential disposed in circuit with said control grid and cathode electrodes of said reactance tube for providing a constant mean value for aid non-modulated frequency of said crystal body, said single pair of electrodes of said crystal body being connected in series circuit relation with said inductance coil in the control gridcathode input circuit of said oscillation generator.

4. Apparatus for varying and controlling the frequency of oscillation of an oscillation generator comprising a piezoelectric crystal body having a. single pair of opposite electrodes, said electrodes comprising the sole efiective capacitance producing means directly shunted across said crystal body, and means including said crystal body controlled by positive reactance means having a definite predetermined control value responsive to the amplitude of variable bias potential applied thereto for correspondingly varying the operating frequency of said crystal body and said oscillation generator, said positive reactance means comprising a variable reactance tube having anode, cathode and control grid electrodes, and an inductance coil disposed in circuit relation across said anode and cathode electrodes of said reactance tube and connected inseries circuit relation with said single pair of electrodes of said crystal body, said inductance coil having a tap connection thereon, a resistor connected at one of its ends to said tap connection on said inductance coil, and a condenser connected between the other end of said resistor and said cathode electrode of said reactance tube, and another condenser connected between said other end of said resistor and said control grid electrode of said reactance tube, said inductance coil having an inductive reactance substantially greater than the anode circuitimpedance of said reactancetube and substantially equal to the static capacitive reactance of said crystal body at said operating frequency of said crystal body, and means including a stable source of constant bias potential disposed in circuit with said control grid and cathode electrodes of said reactance tube for providing a constant mean value for said non-modulated frequency of said crystal body, said single pair of electrodes of said crystal body being connected in series circuit relation with said inductance coil in the control grid-cathode input cir cuit of said oscillation generator, and a seriesconnected condenser in said last-mentioned circuit connected in series circuit relation with said crystal body and said inductance coil for adjusting the mean value of said frequency of said oscillation generator.

WARREN P. MASON.

REFERENCES CITED The following references are of record in the file of this patent:

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